Move Nix files into a nix directory

Move everything from `config` to the root level.

nvim/plugin/oil.lua

@@ -0,0 +1,16 @@
+require("oil").setup {
+  columns = { "icon" },
+
+  keymaps = {
+    ["<C-h>"] = false,
+    ["<M-h>"] = "actions.select_split",
+  },
+
+  skip_confirm_for_simple_edits = true,
+
+  view_options = {
+    show_hidden = true,
+  },
+}
+
+vim.keymap.set("n", "-", "<Cmd>Oil<cr>", { desc = "Open parent directory" })